Electronic Devices: How Black Lights Work

Black lights work like traditional light bulbs, using a few unique materials to create their eerie violet glow.

The black lights that were once only common in dorm rooms and haunted houses are now found everywhere. Popular forensic television shows depict investigators employing them at crime scenes. Antique appraisers use them to spot fakes. At the theme park, they show invisible admission hand stamps. In short, black lights have gone from groovy to useful in our society. Still, almost no one has a clue about how they work. They wonder if they could paint their own lights black to achieve the same glow-in-the-dark effect. As for why they make things glow, people do not have the slightest idea. For the curious, here is an explanation of the science behind the black light.

How They Work

The way a black light works is very similar to a regular fluorescent tube light. In the basic fluorescent lamp, electricity is channeled through a tube filled with inert gas and a small amount of mercury to produce light. The mercury is charged by the electricity, causing it to release photons of light. Because the majority of this light is invisible UV light, a molecular change must take place in order for the lamp to emit sufficient light. To accomplish this, the outer tube is coated with phosphorus. When the UV light hits the phosphorus, it heats us in reaction. Upon cooling, it releases a photon of visible light.

What makes a black tube light different from a fluorescent tube light is its coating. It is designed to absorb the seriously dangerous UV-B and UV-C rays, allowing only the relatively safe UV-A light to escape. In a black incandescent bulb, the coating filters light that the filament creates, letting UV-A rays as well as a bit of visible light escape. Fluorescent black lights are considered the more powerful of the two because of the amount of light produced within the tube. This allows for the creation of more emitted UV-A light, which is responsible for the black light "glow."

Ultraviolet waves from black lights react with the phosphorus in their coatings and the environment. When UV light touches phosphorus, visible light is created. The environment is full of naturally occurring phosphorus: it is in our teeth, nails, and body fluids just to name a few. Although every object containing phosphorus will emit light when lit by a black light, the color of the object dictates how bright it will glow. For instance, black clothing absorbs every color of visible light cast on it, so UV-A rays have little effect on them. On the other hand, white clothes are already bright because they refract every color of visible light. When UV-A rays hit white, it reflects ultraviolet rays converted to visible light in addition to the usual reflected light.

Where They Work

As mentioned above, the body has a heavy supply of phosphorus. In addition, manmade phosphorus is present in all kinds of things. You will find it in laundry detergent, television screens, and highlighter ink. It is useful in searching crime scenes because it makes semen, urine, and other body fluids glow under a black light. In addition, a phosphorus-loaded dust is often sprinkled by investigators to reveal fingerprints. Phosphorus is used for security in any number of situations. For instance, modern paper money has a strip treated with the chemical so that frauds can be detected easily. Antiques can be distinguished from fakes under black light because only modern paint contains phosphorus. None of these methods exposes light operators to serious risk because of the minimal amount of UV rays emitted. However, it is important to understand the potential dangers of handling black lights.

Safety Issues

There are grades of black lights far more powerful than the kind available in novelty shops. These bulbs release high levels of UV-A for varying purposes. One example is found in lab equipment sterilizers, where substantial amounts of ultraviolet rays are released from a black light to destroy germs. Professional rock hunters also use a specialized light that reveals certain valuable phosphorous-rich stones. Black lights of this strength release far more UV-A rays than one can healthily be exposed to. While this form of light is the least infamous of the ultraviolet family, it has been linked to immune system suppression, toughening of the skin, and even cataract development. Even those who use recreation grade black lights should take special precautions. If you are going to be under these lights for a prolonged time, wear protective glasses. Better yet, never point a black light at people, using them instead to enhance phosphorescent displays. Although UV-A rays are too short to see, they can potentially be as harmful as a day of unprotected sunbathing.

The black light is an interesting and highly adaptable tool. When used respectfully, it can do far more than make a retro wall hanging glow. Keep this in mind the next time you write a note with an invisible black light pen or have your repairman find a ventilation leak by injecting phosphorus ink into the system. The "magic" you witness is actually the result of a series of molecular reactions. Those creepy purple bulbs that most people think are useless help prevent fraud, avert illness, and solve crimes.